What first comes to mind as feed for fish farming is raw bait that is also consumed as bait in the natural world (fresh or frozen horse mackerel, mackerel, sardines, sand eels, squid, and the like). Raw bait is originally natural bait for fish, and is therefore excellent in terms of being preferred by fish and the nutritional benefits for fish, but suffers from problems such as stable supply and stable product quality due to being derived from natural resources. Moist pellets, which are obtained by blending vitamins, minerals, natural starches, and the like with raw bait, are superior in terms of nutritional benefits for fish and stability of supply, but problems of environmental pollution caused by moist pellets being discharged into marine environments rather than being consumed by fish have been pointed out. In addition, the use of these feeds involves drawbacks such as storage facility costs due to the need for storage in freezers or refrigerators.
Solid fish farming feeds obtained by using only artificial feed raw materials and molding with a steam pelleter or extruder overcome these drawbacks and are advantageous in terms of stability of feed supply and feed shelf life. As a result, bait given to farmed fish is changing from raw bait to artificial feed in the fish farming industry. However, although switching to artificial feed is essential for lasting fish farming, the preference for raw bait is extremely high in highly carnivorous fish, meaning that switching to artificial feed is not easy for these fish types. For example, Pacific bluefin tuna, Thunnus orientalis, and Northern bluefin tuna. Thunnus thynnus, have an extremely strong preference for raw bait, but have an extremely low preference for artificial feed, meaning that there has been a delay in switching from raw bait to artificial feed for these species. In addition, the switch from raw bait to artificial feed is progressing for Yellowtail, Seriola quinqueradiata, and Greater amberjack, Seriola dumerili, but it is not yet easy to farm these fish using only artificial feed.
A variety of techniques have come to be used in the development of feeds that are acceptable to fish such as these tuna species. Patent document 1 is an invention that focuses on the size of feeds and relates to a method for efficiently producing samples of a size that is preferred by tuna species. In addition, patent document 2 discloses a feed having a certain softness, which is obtained by sealing feed raw materials in an edible film. Furthermore, patent document 3 discloses a feed which consists of a shell that includes a sticky material and a core that contains a nutrient ingredient and which is produced by using a filling-wrapping machine.